for now, don't use group taxons in sightings

I was linking to Group records in clements to record yellow-shafted
versus red-shafted Flicker. For now, put that information into comments
for each sighting, so we can preserve the assumption that all sightings
link to a taxon that actually has a common name
add scope for taxon's seen and photographed
remove species model, replace with taxon model

remove relationships to species model
rewrite helper methods to use taxons
replace species class with taxon class

create same JSON API for taxon that species has
replace species class with taxon class
replace species class with taxon class

remove some tests
rewrite most
fix functional tests

replace species model with taxon model
fix locational views

replace species with taxon
fix views, replace species class with taxon class